So why even bother? For 99% of the people, they won't bother and won't do anything and they don't need to. Nope, not at all. So, let's put that aside. This is about wanting to, not needing to. I fuss with my pickup truck too!! I am a geek about some stuff like this. I merely report the truth, I am not trying to convince anyone of anything, OK?
Wheels and tires, as kits, can be ordered through any Kubota dealer as whole goods. There is some price difference from dealer to dealer, but you won't save a lot by shopping. Mostly you pay list, $300-400, plus whatever shipping you can dicker for.
I almost sprang for the wheels and tires used until last year on the BXs. The fronts were 18" but the wheel was still an 8", just the tire was fatter giving the larger diameter. While not everyone likes the look of the newer 10" front wheel that Kubota changed to, I personally like it. The diameter remained 18", so no diameter change was made, the tire just became a lower profile.
I was also concerned, a little bit, about rear fender clearance in moving up from a 24" to 26" tire. I measured and measured with my shop tools and carpenter's square. Yup, there was enough room. I also fiqured that I could easily re-shape that fender detail just in front of the wheel, if I needed to. Turns out, it isn't an issue. Bolt patterns are identical of course, because the hubs are identical in all BXs.